[MikeD]

68" is the combination of medium limbs and 25" riser. I don't know the actual length of the string before and after the stretch. 62.5" rings a bell. But it then had a few twists, which would shorten it a bit.

However I think Pythagorus shows it would've streched about 2.47mm ish as the change in brace height was from 22cm to 21cm.

That sounds reasonable. Could it be the string settling inside the serving?

I knew geometry would come in useful some time. Only took 30 years after school.